That sucks however its a more humain death than nature can offer. Let's see what are the alternatives. Starvation,Predation,wounded by vehicle, wounded by other deer. The human is the only predator that cares about making a quick kill. Other preditors dont care. When a pack of wolves are tearing apart a calf alive in front of his mother i dont think they care about a quick kill. Most drivers that injure a deer are usually on the phone with the insurance company after the collision not looking in the woods to make sure the deer is dead. Starvation really suck it is a slow process till the deer finally falls over and dies. I forgot to mention freezing this is a pretty bad way to go. Anyway don't beat yourself over it. That deer is actually lucky that you put it out the alternatives are worse Orly